As a verb, this is when an individual uses the theme from The Fresh Prince of Bel Air as a lead-in to a story or conversation, often a bad one. This is akin to the idea of the Bel Air, but instead lulls the listener into a false sense of security then drops the bomb on their head. This concept was first found in the webcomic xkcd, #464, entitled RBA.

This is how a Reverse Bel Air should work:

Girl: Now this is a story all about how, my life got flipped turned upside down, and I'd like to take a minute, just sit right there, I'll tell you about how I became uncertain about our relationship. I think you just like having a girlfriend, it doesn't matter who. I think we should break up.
Guy: ...wait, seriously?
Girl: Yeah.

When you begin a conversation by reciting the opening rap from the Fresh Prince of Bel-Air, but finish with something else, for example, a plea to break up.

Now this is a story all about how my life got flip turned upside-down. I'd like to take a minute, just sit right there, I'll tell you how I was confused by a comic and had to submit a new definition for reverse bel air to urban dictionary.

The Urban Dictionary T-Shirt

Using the Fresh Prince of Bel-Air theme song to break up with someone. (As seen in www.xkcd.com/464)

Now, this is a story all about my life got flipped turned upside down and I'd like to take a minute just sit right there. I'll tell you how I became uncertain about our relationship. I think you just like having a girlfriend, it doesn't matter who. I think we should break up.
The Reverse Bel-Air only works once, so make it unforgettable.

start a conversion, but you use
the lyrics from the theme of a song beyond compare
the theme song to the show fresh prince of Bel Air

Now this is a story all about how my life got filliped-turned upside down and I'd like to take a minute, just sit right there I'll tell you how I added music to an urban dictionary definition for reverse bel air but fail it in the example

The Urban Dictionary T-Shirt

Now this is the story all about how my life got flipped, turned upside down and I'd like to take a minute just sit right there. I'll tell you how I became the "Reverse Bel Air" - a story that starts with the first few lines of the lyrics to "Fresh Prince of Bel-Air," but suddenly changes to something shocking, unforgettable, and unrelated to the Prince of Bel Air.

That dude posted about the Fresh Prince of Bel Air on a forum but he RBA'd (Reverse Bel Aired) into a discussion about his gf.